Radar Abstract: The RADAR study was launched in 2014 to provide critical insights into risk and protective factors for HIV and drug use and to identify facilitators and barriers to the implementation of an effective strategy to end the HIV epidemic. Specifically, RADAR seeks to understand the connections between sexually transmitted infections (STIs), substance use, and romantic relationship patterns over time among young men who have sex with men (YMSM) and transgender women. During our initial grant period, we were extremely productive in terms of recruitment, retention, scientific output, and training future scholars. RADAR recruited over 1,200 study participants and maintained a 80% retention rate since it was launched. To date, more than 40 scientific publications have been published. RADAR also served as a platform for innovative research, with more than 25 connected studies and seven concepts via the NIDA C3PNO consortium. The project utilizes a multilevel research design that incorporates biological, dyadic, and network components to provide an unprecedented view into HIV transmission epidemiology. Cohort members are assessed every six months on individual behavior and risk; relationship and sexual partner characteristics; and social, sexual, and drug-using networks. Members also receive multiple biological assessments (drug and STI testing and banking of plasma and DNA) at each six-month assessment. The RADAR cohort includes both HIV-positive and -negative participants, and it continues to build a repository of HIV sequence data and biospecimens from time points before, during, and after acute infection that can facilitate future proposals evaluating substance use, HIV risk, pathogenicity, and immunity.